Heidegger on Ontological Difference

Heidegger's magnum opus, Being and Time (1927), was never completed, but the reasons underlying this 'incompletion' remain unclear. The central point is that Heidegger's guiding question––viz., the question of the meaning of Being––cannot be answered merely by describing the ontological constitution of one being: the human being, or Dasein. This is how Heidegger himself interprets the 'incompletion' of Being and Time in his later writings. What is it about Dasein that makes it a necessary, but insufficient condition under which to raise the question of the meaning of Being?